1. Field of the Invention
The invention relates to a circuit for providing standby operation in a switched power supply with a blocking oscillator.
Equipment in entertainment electronics requires working and rest operations which should be capable of being switched-over without a mechanical on-and-off power switch. The rest operation is also called standby operation. The standby operation requires a minimal power loss, high efficiency, inaudible operation as well as the ability to be switched by remote control.
2. Description of the Related Art
In modern equipment provided with a blocking oscillator, the above-mentioned conditions have been met heretofore either by providing a separate standby-operation power supply and by disconnecting the blocking oscillator with a relay on the primary side of the power supply transformer, or by means of a self-oscillating or free-running blocking oscillator, where the normal load in the working operation is disconnected by a relay, and a control circuit is provided for operation with a minimum load in standby operation.
However, the no-load voltages on the secondary side increase so that destruction could occur in the event of a disturbance on the primary side.
The structure of such a blocking oscillator-switched power supply is described in more detail below and in German Published, Non-Prosecuted Application DE-OS No. 33 12 209, corresponding to U.S. Pat. No. 4,593,347.
Every modern blocking oscillator-switched power supply is constructed in such a way that it can be operated not only in normal operation but with appropriate protective circuits in overload or short-circuit operation as well. In prior art devices for providing standby operation, very high frequencies of 70 kHz, for instance, are obtained for standby operation, due to the load behavior on the secondary side. The disadvantage of such operation is that it leads to power losses in the transformer as well as to further power loss contributions due to the charge reversal of capacities.
It is accordingly an object of the invention to provide a circuit for providing standby operation in a switched power supply with a blocking oscillator, which overcomes the hereinafore-mentioned disadvantages of the heretofore-known devices of this general type and which permits standby operation at lower frequencies.
With the foregoing and other objects in view there is provided, in accordance with the invention, a circuit for providing standby operation in a switched power supply with a blocking oscillator, comprising a control circuit, an electronic switch driven by the control circuit, and a transformer having a primary winding connected in series with the electronic switch, a first secondary winding supplying power supply to a normal load of electrical equipment, a second secondary winding driving the control circuit, and a further transformer winding strongly or permanently coupled to the primary winding and loosely coupled to the first secondary winding for generating a control signal for a standby load supply.
In accordance with another feature of the invention, there is provided a comparator stage coupled to the further transformer winding, to a reference and to the standby load for supplying the standby load.
In accordance with a further feature of the invention, there is provided a switch connected to the first primary winding, to the comparator stage, to the normal load and to a further standby load, the comparator stage controlling the switch for decoupling the normal load from the first secondary winding as well as for supplying the further standby load.
In accordance with a concomitant feature of the invention, the control circuit includes a pulse width modulator, a control voltage generator, and an overload amplifier, the overload amplifier being driven by the control voltage generator when the normal load is decoupled and the overload amplifier in turn controlling the pulse width modulator for driving the electronic switch for generating a pulse-shaped control signal with a duty cycle of less than 1:10 and preferably about 1:100 and a frequency above a given audibility limit.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in a circuit for providing standby operation in a switched power supply with a blocking oscillator, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.